A sequence ana_nan is arithmetic. If a4+a8=30a_4 + a_8 = 30a4+a8=30 and a2+a10=30a_2 + a_{10} = 30a2+a10=30, which of the following is true?
a6=15a_6 = 15a6=15
d=0d = 0d=0
a1=5a_1 = 5a1=5
ana_nan is constant